Nitrided alloy steel cold-drawing die



Patented Nov. 3, 1936 j .UNITED STATES PATENT OFFICE I William'Herbert Hatfield, Hathersage, England; assignor to Thos. Firth & John Brown Limited, Sheilield, England, a British company No Drawing. Application July 24, 1934, Serial No. 736,785. In Great Britain July 25, 1933 Claims. (01. 148-31) This invention comprises improvements in or containing 8 to 25% tungsten, an efiective relating to alloy steels; particularly for coldamount up to 1% carbon, an effective amount drawing dies; up to 8% chromium, an efiective amount up to It is well known that for dies used, in cold- 3% vanadium and, if desired, an efiective amount 5 drawing operations for the production of wire, up to 20% cobalt and/or an effective amount up tubes, and special sections of metals and alloys, to 8% molybdenum and normal proportions. of a hard surface resistant to wear is necessary, other elements such as silicon and manganese, since the hardness and resistance to wear afiect and modified by the further addition of an efthe life of the die, the uniformity of dimensions fective amount up to 2% aluminium. By the use 10 and properties of the cold-drawn articles, and the of such an alloy a core hardness of the order of 10 amount of frictional resistance to passage 550 Brinell or over can readily be obtained. through the die. While the invention is particularly concerned Alloy steels which have been surface-hardened with the fabrication of cold-drawing dies, it inby the so-called nitrogenizing process have aleludes alloy steels (for whatever purpose they ready been used for cold-drawing dies. In the may be subsequently used) which have the par- 15 nitrogeni'zlng process the alloy steel is subjected ticular composition set out above and which have to "the action of gases containing nitrogen in -been subjected to the nitrogenizing process. 7 combined form (e. g. ammonia gas) at a tempera- It is known that many high-speed tool steels ture in the region of 500 C. The hardness of the are capable of a limited degree of surface-harden- :0 core material of those alloys after nitrogenizing, i by nitrogenizing but, according to the present which is practically the same as the general hardinvention, such surface-hardening is rendered ness of the alloy before nitrogenizing, is. limited more intense or'of more practicalservice by the by the nature of the steel and by the fact that'thje .incorporation of aluminium, in the proportions prior treatment is such that a stable conditionof "defined above, in the steel.

the core material is reached which is unafiected .It"..is to be noted that the present invention 25 subsequently by the temperature employed inlthe ds-concerned solely with the manufacture and use nitrogenizing operation. This prior treatment" of ajnitrogenized high-speed toolsteel embodying usually consists either of normalizing or hardenasan-essential constituent up to 2.0% aluminium. ing and tempering, and the hardness of the ma- Thei application of the nitrogenizing process to terial when ready for the surface-hardening alloy steels of other types which may contain 30 treatment usually lies between about 140 and aluminium has already been described. Forex- 350 on the Brinell scale. The surface hardness ample, British specification No. 174,580 describes after nitrogenizing may be as high as 1100 to the 'nitrogenizing of steel alloys containing 1150 on the diamond hardness scale. aluminium, chromium, manganese, or silicon In practice cold-drawing dies made from either separately or in any desired combination.

nitrogenized alloy steels have, in certain cases, Again in British specification No. 243,613 it is been too weak to stand up to the heavy pressures stated that articles which have their surface involved in the drawing operations, owing to the layers hardened by nitrogenization may assumefact that the thinnitrogenized skin has transowing to the heating required for carrying out the 40 mitted the pressure to the softer and more readily operation of nitrogenization and the subsequent 40 deformable core. The principal object of this ingradual cooling-a temper-brittle condition of vention is to provide an improved cold-drawing the unhardened core-zone, and this condition can die which does not suffer from the above defect, be remedied by using steel alloys which contain and which at the same time provides a hard 0.5 to 2% of aluminium and, either separately wear-resisting surface. This object is achieved or in any desired combination, silicon, manganese, 5 by fabricating the die from an alloy steel of such nickel, chromium, molybdenum, tungsten, vaa constitution that it is capable of surfacenadium, titanium or zirconium amounting in all hardening by nitrogenizing and at the same time to from 0.5 to 4% and up to 0.6% of carbon. It provides a body portion or core which may be, has also been proposed to apply the process of and shall remain after completion of the nitronitrogenizing at a temperature of 550 to 950 50 gizini process, sufliciently hard to be for all prac- 'C. to alloy steels of austenitic structure, and in tical purposes undeformable. some examples embodying that proposal the According to the invention cold-drawing dies austenitic alloys contained up to 7% of alumiare made from a nitrogenized alloy steel having um. It is to be observed that none of these'prothe normal composition of a high-speed tool steel posals is concerned with the nitros n f an alloy steel having the composition of a high-speed tool steel, nor with the use of such steels in colddrawing dies.

In carrtdng out the present invention the tool steels, prior to nitrogenizing, may conveniently be treated by quenching or quickly cooling in oil, air or water from a high temperature, e. g. 1150 C., and tempering at ateruperature not below 550 (3., giving the well-known red hard condition. if desired, the tempering may be omitted as a separate process since the -subsequent nitrogenizing process at a temperature in the region of 500 C. will, to a large degree, accomplish thetempering of the core material without undue softening and, at the same time, produce a superhard skin.

A specific steel within this invention, on an= alysis showed the following composition:-

- Percent Carbon 0.30 Silicon 0.22 Manganese 0.24 Chromium 3.13 Nickel 0.12 Tungsten 10.27 Vanadium 0.5a Aluminium 1.21

Depth be Hardness low surface V. D. H in mms. No.

In order to obtain the required hardness of the core, prior to nitrogenizing, the steel was 011 quenched from 1150 C. and tempered at 580 C.

for one hour. It is noteworthy that the hard- I ness of the core remains unchanged by the nitrogenizing treatment.

I claim:-

1. A cold-drawing die made from a. nitrogenized alloy steel containing 8 to 25% of tungsten, an efiectlve amount up to 1% of carbon, an effective amount up to 8% of chronuurn, an efiective amount up to 3% of vanadium, an e1"- iective amount up to 20% of cobalt, and modifled by the further addition of an effective amount to 2% of aluminium, the balance being iron together with such other alloying elements and impurities as are usual in alloy ste the die being characterized by having a core-l ness of the order of 550 Brinell and a s hardness of upwards of 1100 on the diamond hardness scale.

2. A cold-drawing die made from a nitrogen ized alloy steel containing 8 to 25% of tungsten, an efiective amount up to 1% of carbon, an eh fective amount up to 8% of chromium, an effective amount up to 3% of vanadium, an eiiective amount up to 20% of cobalt, an effective amount up to 8% of molybdenum, and modified by the further addition of an effective amount to 2% of aluminium, the balance being iron together with such other alloying elements and impurities as are usual in alloy steels, the die being characterized by having a core-hardness or" the order of 550 Brinell and a surface-hardness of upwards of 1100 on the diamond hardness scale.

3. A cold-drawing die having the core portion of a nitridable steel alloy containing tungsten from about 8 to about 25%, carbon from about 0.2 to about 1%, chromium from about 2 to about8%, vanadium in an effective amount up to 3%, aluminium from about 0.2 to about 2%, and the remainder principally iron, said die having a corehardness of the order of 550 Brinell even after being subjected to a temperature of about 500 C. and the working part having a surface-hardened by nitriding of the order of 1100 Brinell.

4. A process of surface hardening a cold drawing die which comprises nitriding a cold drawing die of high speed tool steel containing 8 to 25% of tungsten, an effective amount up to 1% of carbon, an effective amount up to 8% of chromium, an effective amount up to 3% of vanadium, an effective amount up to 2% of aluminium, the balance being iron together with such other alloying elements and impurities as are usually found in alloy steels; and treating the said steel prior to the nitriding by quenching from a temperature of the order of 1150 C.

5. The process set forth in claim 4, in which the steel is tempered at a temperature of the order of 550 C. after the step of quenching from 1150 C.

WILLIAM HERBERT HATFIELD. 

